The present invention relates to an upstream signal control apparatus used in a bidirectional CATV system which allows communication between subscriber terminals and the broadcast center.
A conventional bidirectional CATV system sends television and FM signals (including data service and audio signals) to a large number of subscribers through cables and receives upstream signals from subscriber terminals in a time-division scheme. Each subscriber terminal sends back an upstream signal during a certain period of time (an upstream sending period will be referred to as a polling period hereinafter). The upstream signal includes information for reserving desired pay programs, information representing an answer to a question from the broadcast center, information representing debt incurred by watching pay programs, security information, and the like. The upstream signal is generated by an upstream signal control apparatus in the subscriber terminal. The data transmission time (polling period) of the upstream signal control apparatus is normally set to be very short. This is because the broadcast center must time-divisionally communicate with many subscriber terminals in the CATV system.
A critical problem occurs in a bidirectional CATV system when the upstream signal control apparatus signals out of turn and an upstream signal is sent onto the cable irrespective of a request from the broadcast center. In this case, other subscribers' upstream signals are disturbed, and the broadcast center cannot receive them. For reasons that should be obvious it is necessary to prevent generation of a data carrier in the subscriber's terminal during all but a specifically assigned polling period. To this effect the known modulating schemes are those of FSK, PSK and ASK.
In a conventional CATV system, the level of upstream signals from different subscriber terminals must be uniform at the broadcast center. However, as the physical locations of the subscriber terminals differ, if the transmission gains of the upstream signal control apparatuses are initially set to be identical, the above requirement cannot be satisfied. Therefore, transmission gains of the upstream signal control apparatuses must be adjusted to optimal values in accordance with terminal distance from the broadcast center and trunk amplifiers.
Transmission gain adjustment of an upstream signal control apparatus is performed during the polling period. For this reason, installation personnel must adjust transmission gains while they check transmission level monitoring equipment. Moreover, the transmission gain adjustment period itself is very short as it must be completed within the polling period. When satisfactory adjustment cannot be performed within the first polling period, the transmission gain must be adjusted during the next one or more periods. Therefore, transmission gain adjustment of the upstream signal control apparatuses is time-consuming and inconvenient.